Hydraulic damper (also called hydraulic retarder) is an energy-absorbing device converting mechanical energy into heat energy. It is especially employed in automotive auxiliary brake. Two adjacent impellers, i.e. a driven rotor impeller and a fixed stator impeller, are arranged opposite so as to form a working chamber in which a flow loop is formed by liquid medium. The liquid medium is driven by the rotor impeller to rotate around an axis; meanwhile, the liquid medium is moved along the direction of the vanes of the rotor impeller, thrown towards the stator impeller. Due to the reaction force exerted on the liquid medium by the stator impeller, the liquid medium flows out of the stator impeller and turns back to impact the rotor impeller, which forms a resistance torque to the rotor impeller, and hinders the rotation of the rotor impeller, thus achieving a retarding brake on a gear shaft.
When the hydraulic damper works, some liquid medium will be vaporized and converted into gaseous medium with an increased temperature of the liquid medium in the working chamber. If the temperature is high during the operation of the hydraulic damper, especially when the vapor pressure of the employed liquid medium is large, it must manage to discharge the generated gaseous medium, otherwise, the hydraulic damper will be damaged or even exploded due to the excessive pressure in the working chamber.
Since there is no clear dividing line between the liquid medium and the gaseous medium in the working chamber when the hydraulic damper works, in most cases, the liquid medium coexists with the gaseous medium in the working chamber. If the gaseous medium is needed to be discharged out of the working chamber, some liquid medium may inevitably be discharged, which resulted in an excessive loss of the liquid medium in the working chamber.